1901-2.] J. G. Goodchild on Scottish Mineralogy. 335 
Cerussite . — An exceptionally fine series of crystals of Scottish 
Cerussites, nearly all from the Leadhills district, is exhibited in the 
Scottish Mineral Collection. Few, if any, of these appear to 
have been figured, In going through the Collection, chiefly with 
the object of determining and recording the various combinations 
represented, I made freehand drawings of a large number of the 
crystals. The individual crystals so drawn are clearly indicated 
by means of pointers attached to the specimens in which they 
occur, or else are mounted with seccotine on small corks, which, in 
turn, are fastened to the card on which the sketch has been made. 
Registration marks readily enable one to refer to, or to find, the 
specimens so figured. This plan has been followed throughout the 
whole of the Collection. 
Cerussite, the Orthorhombic Lead Carbonate, is always a 
mineral of secondary origin, and has generally arisen from the 
decomposition of other ores of lead through the action of waters 
percolating from the surface. Galena is the^usual parent of Cerussite. 
The active solvent agent appears to have usually been either car- 
bonic acid or else one or other of the humus acids. It is worthy of 
remark, therefore, that Cerussite arises only under humid climatal 
conditions. In those regions where there are deserts, or where, 
in general, arid conditions obtain, it is probable that the carbonates 
are not so commonly formed as decomposition products of metal- 
liferous deposits as are the chlorides and bromides. A review of 
the regions of the world where the chlorides and bromides of the 
metals are of most common occurrence will at once show that 
these regions are geographically regions of inland drainage, where 
the rate of evaporation either balances, or is sometimes in excess of, 
the precipitation. Under these conditions the chlorides which are 
carried inland from the sea, first in the form of sea-spray, and then 
of salt dust, eventually form the nuclei of raindrops, of flakes of 
snow, or of dew and hoarfrost, and are left upon the land as 
evaporation carries off their solvent. The surface soil thus becomes 
heavily impregnated with these desiccation products, which, under 
normal geographical conditions, would have been returned to their 
native home, the sea, but which, under arid climatal conditions, 
slowly percolate into the crust of the earth, and set up chemical 
changes which may eventually become of considerable importance. 
